This subreddit is a work in progress, it will eventually become a massive list of every active subreddit, sorted by genre(s). You can also submit links to subreddit lists and other resources outside of reddit. Hopefully someone will take those resources and make a self post here.

For example, you want to make a list of subreddits about books. First, make self post entitled "Book Subreddits". Include every subreddit that you know of that involves books. If other users know of a book related subreddit that isn't on the list, they should leave a comment with a link to the subreddit. Hopefully the OP will see the comment and add it to the list.

Checking back at least once every week or so should be fine. Feel free to create a new list if the original has been abandoned (tons of new subreddits in comments, the list hasn't been updated in weeks).